chapter 4.2 building near trees

Standards

Chapter 4.2Building near trees

April 2003 editionEffective from July 2003

This chapter gives guidance on meeting the TechnicalRequirements and recommendations for building neartrees, particularly in shrinkable soils.

NATIONAL HOUSE-BUILDING COUNCIL

CI/SfB

CAWS

81 11

| (H1)

D10

NHBC

INTRODUCTION4.2 Building near trees

THE STANDARDS

The NHBC Standards give:

• Technical Requirements in red

• Performance Standards in dark blue

• Guidance in light blue

for the design and construction of dwellings acceptableto NHBC. The Technical Requirements are given inChapter 1.1 and these must be met by the builder.

Diagrams may contain text in red. This is to highlightpoints but has no mandatory significance.

The Standards come into effect for every NHBCregistered home whose foundations are concreted onor after the date shown on the cover of each Chapterand apply throughout the UK, unless otherwise stated.

COMPOSITION OF THE STANDARDSThe Standards are divided into 10 Parts, eachcovering a particular aspect. The Parts follow theusual construction process.

In general, each Chapter is made up of sectionsdealing with Design, Materials and Sitework.

In some cases one or more of these aspects may notbe included.

SCOPEThis Chapter gives guidance on meeting the TechnicalRequirements and recommendations when buildingnear trees, hedgerows and shrubs, particularly inshrinkable soils.

NHBC Standards do not cover aspects of health andsafety relating to building operations and to thehandling and use of certain building materials. Suchmatters are covered by statutory requirements.

FINDING INFORMATIONThe following example gives guidance on how to findinformation on a particular subject:

For example: 4.2 - D5(b) means:

4 Part 4 Foundations

2 Chapter 2 Building near trees

D Section DESIGN

5 Clause 5 FOUNDATIONS (shrinkable soils)

(b) Item (b) soil classification

INTRODUCTION 1

DESIGNDesign standard D1 2

Statutory requirements D2 2

Trees and hedgerows adjacent

to structures D3 2

Foundations (all soil types) D4 2

Foundations (shrinkable soils) D5-D7 3

Designing to accommodate heave D8 8

Provision of information D9 10

MATERIALS

Materials standards M1 11

Proprietary heave materials M2 11

SITEWORK

Sitework standards S1 12

Foundation depths S2 12

Excavation for foundations S3 12

Heave precautions S4 14

Drainage S5 16

APPENDIX 4.2-AStatutory references 17

APPENDIX 4.2-BWater demand and mature height of trees 18

APPENDIX 4.2-CFoundation depth charts 19

APPENDIX 4.2-DFoundation depth tables 23

APPENDIX 4.2-EClimate zones 29

APPENDIX 4.2-FDamage to trees by construction work 30

APPENDIX 4.2-GInformation sources and acknowledgements 33

APPENDIX 4.2-HWorked example 34

LIST OF CHAPTERS 37

INDEX 37

Introduction NHBC Standards - Chapter 4.2Effective: July 2003 April 2003 edition

CONTENTS Clause Page

INTRODUCTIONBuilding near trees 4.2

INTRODUCTIONThe combination of shrinkable soils and trees,hedgerows or shrubs represents a hazard tostructures that requires special consideration. Trees,hedgerows and shrubs take moisture from theground and, in cohesive soils such as clay, this cancause significant volume changes resulting in groundmovement. This has the potential to affectfoundations and damage the supported structure. Inorder to minimise this risk, foundations should bedesigned to accommodate the movement or be takento a depth where the likelihood of damagingmovement is low.

This Chapter gives guidance for common foundationtypes to deal with the hazard and includes suitablefoundation depths which have been established fromfield data, research, NHBC data and practicalexperience. The depths are not those at which rootactivity, desiccation and ground movement are nonexistent but they are intended to provide anacceptable level of risk. However, if significantquantities of roots are unexpectedly encounteredin the base of the trench, the excavation may needto be deepened.

The interaction between trees, soil and buildingsis dependent on many factors and is inherentlycomplex. The relationship becomes less predictable asfactors combine to produce extreme conditions.These are signified by the need for deeperfoundations. Depths greater than 2.5m indicatethat conditions exist where prescriptive guidanceis less reliable.

The following situations are beyond the scopeof the guidance in this Chapter and will requirea site specific assessment by an Engineer(see Technical Requirement R5):

• foundations with depths greater than 2.5m withinthe influence of trees

• ground with a slope of greater than 1 in 7(approximately 8°) and man made slopes suchas embankments and cuttings

• underpinning.

Consideration has been given to the potential effectsof climate change in the guidance provided.

The services of a specialist arboriculturalist may behelpful for the identification of the type andcondition of trees that may affect building work.This includes trees both on and adjacent to the site.

NHBC Standards - Chapter 4 2 Page 1 of 37

April 2003 edition Effective: July 2003

DESIGN4.2 Building near trees

DESIGN STANDARD4.2 - D1 Design shall meet the Technical

RequirementsDesign that follows the guidance below will beacceptable for building near trees, hedgerowsand shrubs.

STATUTORY REQUIREMENTS4.2 - D2 Design shall comply with all relevant

statutory requirementsA list of statutory references applicable to thisChapter is given in Appendix 4.2-A.

TREES AND HEDGEROWSADJACENT TO STRUCTURES4.2 - D3 The design shall take account of trees

and hedgerows and their growth

Items to be taken into account include:

(a) removal of existing trees and hedgerowsDead trees and dead hedgerows should be removed.Unstable trees should be made stable but where thisis not possible they should be felled. If in doubt,advice should be obtained from a RegisteredArboriculturalist.

Acts of Parliament, planning conditions, conservationarea restrictions or tree preservation orders may meanthat trees and hedgerows are protected and must beretained. The local planning authority shouldbe consulted.

(b) protection of remaining trees and hedgerowsMost of a tree's root system is within 600mm of thesurface and extends radially for distances often inexcess of the tree's height. All parts of the rootsystem are vulnerable to damage and once damaged,roots may not regenerate. Extensive root damagemay impair the stability of the tree.

Root damage and tree instability can be caused by:

• stripping topsoil too close to trees

• excavating trenches for foundations and servicestoo close to trees

• raising soil levels adjacent to trees, particularlywhere non-granular materials are used

• compaction of soil around trees by heavy plant

• storage of heavy materials around trees

• covering rooting area with impervious surfaces.

Damage should be avoided by erecting suitablefencing to create a tree protection zone for eachremaining tree and ensuring this zone is leftundisturbed during construction work.

Further guidance is given in BS 5837 andAppendix 4.2-F.

(c) allowance for physical growth of young treesDirect damage due to the growth of the main trunkand roots of young trees should be avoided bylocating structures and services at a safe distancefrom the trees. Further guidance is given in BS 5837and Appendix 4.2-F. Where this cannot be achievedprecautions should be taken to allow for futuregrowth. For example:

• foundations should be reinforced to resistlateral forces

• walls or structural slabs should bridge over theroots allowing sufficient clearance for futuregrowth or be reinforced to avoid cracking

• pavings and other surfaces should be laidon a flexible base to allow for some movement.

FOUNDATIONS (all soil types)4.2 - D4 Foundations for all soil types shall

be designed to transmit loads tothe ground safely and withoutexcessive movement

Foundations for all soil types should be designed andconstructed in accordance with Chapter 4.1 'Landquality - managing ground conditions' and otherrelevant Chapters of the Standards (depending onsite specific conditions).

Different foundation types should not be used tosupport the same structure unless the foundationsand superstructure design are undertaken by anEngineer (see Technical Requirement R5).

The remainder of this Chapter gives additionalguidance that applies when building near treeshedgerows and shrubs on shrinkable soils as definedin Clause D5(b).

Page 2 of 37 NHBC Standards - Chapter 4.2

Effective: July 2003 April 2003 edition

DESIGN

Alternatively the Plasticity Index may be used withoutmodification. For pure clays and other soils with100% of particles less than 425 m the result will bethe same. However, for mixed soils such as glacialtills, use of the modified Plasticity Index may result ina more economic design.

For further information about the modified PlasticityIndex refer to BRE Digest 240.

The volume change potential should be establishedfrom site investigation and reliable local knowledgeof the geology.

Sufficient samples should be taken to provideconfidence that the test results are representative ofthe soil volume change potential for the site. If indoubt use the higher value of volume changepotential.

If the volume change potential is unknown, highvolume change potential should be assumed.

(c) water demand of treesWater demand varies according to tree species and size.

Appendix 4.2-B gives the water demand categoriesof common tree species.

Where the species of a tree has not been identified,high water demand should be assumed.

Where the species of a tree has been identified but isnot listed, the following assumptions may be madefor broad leafed trees:

• high water demand - all Elms, Eucalyptus,Hawthorn, Oaks, Poplars and Willows

• moderate water demand - all others

Where trees are not listed in Appendix 4.2-B,information may be obtained from suitablealternative authoritative sources (see Appendix 4.2-G).

Tree identification can be assisted by reference to atree recognition book (see Appendix 4.2-G).

For the purposes of this Chapter, the zone (i.e. lateralextent) of influence of trees is shown in Table 2.

Table 1 Volume change potential

Modified Plasticity Index Volume change potential

40% and greater High

20% to less than 40% Medium

10% to less than 20% Low

Table 2 Zone of tree influence

Water demand Zone of influence

High 1.25 x mature height

Moderate 0.75 x mature height

Low 0.5 x mature height



FOUNDATIONS (shrinkable soils)4.2 - D5 The design shall make allowance for

the effect of trees and hedgerowson shrinkable soils


(a) shrinkage and heaveShrinkable soils are subject to changes in volume astheir moisture content is altered. Soil moisturecontents vary seasonally and are influenced by anumber of factors including the action of tree roots.The resulting shrinkage or swelling of the soil cancause subsidence or heave damage to foundations,the structures they support and services. Heaveprecautions are described in Clause D8.

Shrinkable soils are widely distributed throughout theUK. Local geological survey maps may give relevantinformation.

(b) soil classificationFor the purposes of this Chapter, shrinkable soils arethose containing more than 35% fine particles andhaving a modified Plasticity Index of 10% or greater.

Fine particles are defined as those having a nominaldiameter less than 60µm, ie. clay and silt particles.

The Plasticity Index (Ip) of a soil is a measure of itsvolume change potential and is determined byAtterberg Limits tests. These tests are carried out onthe fine particles and any medium and fine sandparticles. Soil particles with a nominal diametergreater than 425 µ m are removed by sievingbeforehand. The percentage of particles smaller than425 mm m is routinely reported for Atterberg Limits tests.This is a requirement of BS 1377, which specifies thetest procedure.

The Modified Plasticity Index (I'p) is defined as thePlasticity Index (Ip) of the soil multiplied by thepercentage of particles less than 425 µ m.

i.e. I 'p = Ip x % less than 425 µ m100%

Modified Plasticity Index is related to volume changepotential as shown in Table 1.

Building near trees 4.2

Sharon Smith

Sharon Smith

DESIGN

FOUNDATIONS (shrinkable soils)4.2 - D5 (continued)

(d) tree heightsMature heights of common tree species are listed inAppendix 4.2-B. For the purposes of this Chapter,these are the average mature heights to whichhealthy trees of the species may be expected to growin favourable ground and environmental conditions.These may be used even when the actual heights aregreater.

The mature heights given in Appendix 4.2-B shouldbe used for trees that are to remain or are scheduledto be planted and where ground levels are unaltered.Where ground levels are increased see also Figure 1and Sitework clause S3(c).

Where there are different species within hedgerows,the mature height of the species likely to have thegreatest effect should be used.

For trees which have been or are to be removed,allowance should be made for the fact that the waterdemand of a tree varies with its size and rate ofgrowth (see Figure 1). The water demand of a semi-mature tree may be as great as that for a mature treeof the same species whereas the water demand for asapling or young tree will be significantly less.

Figure 1 Tree height H to be used for particulardesign cases

(e) climateHigh rainfall reduces moisture deficits caused by treesand hedgerows, and cool damp weather reduces therate of water loss from the tree, thus reducing therisk of soil movement. As the driest and hottestconditions in the UK usually prevail in southeastEngland, the greater risk occurs in that area anddiminishes with distance north and west.

For the purposes of this Chapter, the UK has beendivided into zones at 50 mile intervals from London.After the foundation depth has been derived fromAppendix 4.2-C or 42-D a reduction of 0.05m(50mm) may be made for every 50 miles distancenorth and west of London (see Appendix 4.2-E).

4.2 - D6 Foundations shall be capable ofaccommodating the effects of treesand hedgerows on shrinkable soilswithout excessive movement


(a) foundationsFoundations to all permanent structures (includinggarages, porches and conservatories) should takeaccount of the effects of soil desiccation causedby previous or existing trees and trees which arescheduled to be planted.

The following foundations will be acceptable inshrinkable soils, provided that they are capable ofsupporting the applied loads without unduesettlement, heave precautions are taken as in ClauseD8 and their design takes account of Clause D7:

strip

trench fill

pier and beam

pile and beam

raft.

Variations to the foundation depths derived from thisChapter may be permitted where other foundationdepths are traditionally acceptable or where necessaryto take account of local ground conditions, providedthat they can be supported by a design in accordancewith Technical Requirement R5.

Root barriers are not a reliable means of reducing theeffects of trees on foundations in shrinkable soils andare not an acceptable alternative to the guidancegiven in this Chapter.

Freestanding masonry walls should be constructedon foundations in accordance with this Chapter orbe designed to accommodate likely groundmovement, for example, by careful use of movementjoints and reinforcement.

mature height

In this range useH = mature height

as listed inAppendix 4.2-B

50% mature height

In this range useH = actual height

This guidance should be used when:deriving foundation depths when trees have beenremoved (use tree height at time of removal)checking the appropriate level from which depthsshould be measured when trees remain and groundlevels are increased (use tree height at time ofconstruction relative to original ground level)

• determining whether heave precautions should beprovided (use tree height at time of construction).

Where trees have undergone or are to undergo heavycrown reduction or pollarding, the mature heightshould be used or a Registered Arboncuturalist shouldbe consulted to undertake a site specific assessment.


Effective: July 2003 - April 2003 edition

4.2 Building near trees

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DESIGNBuilding near trees 4.2

(b) method of assessment of foundation depthsOne of the following methods may be used:

• design in accordance with this Chapter toa depth derived from Appendix 4.2-C or 4.2-Dtaking account of:

- the site investigation

- the soil volume change potential

- the water demand of the tree

- the appropriate tree height

- the distance of the tree(s) from the foundations

- the geographical location of the site north andwest of London

• appropriate heave precautions.

Note: the most onerous conditions should beassumed in the absence of any of the aboveinformation.

• design by an Engineer in accordance with TechnicalRequirement R5, taking account of:

- the recommendations of this Chapter

- results of the site investigation

- advice, when necessary, from a RegisteredArboriculturalist or other competent personwhose qualifications are acceptable to NHBC.

Note: when this method is used and it results infoundation depths or other details less onerous thanthose derived from this Chapter, the design should besubmitted to NHBC for approval prior to workcommencing on site.

(c) distance between tree and foundationThe distance D between the centre of the trunk and thenearest face of the foundation should be used to derivethe foundation depths from Appendix 4.2-C or 4.2-D.

For trees which have been or are to be removed fromwithin 2m of the face of the proposed foundationand where the height on removal is less than 50% ofthe mature height given in Appendix 4.2-B, it may beassumed that D = 2m.

Note: This is to avoid the anomalous situation where,for example, a "sapling" removed from thefoundation line would otherwise require anunnecessarily deep foundation since the D/H valuewould always be zero regardless of the height H ofthe tree.

(d) foundation depths related to proposedtree planting

Foundation depths relating to proposed tree plantingshould be based on one of the following:

foundation depths derived in accordance withAppendix 4.2-C or 4.2-D, or

foundation depths shown in Table 3 withlimits agreed in the planting schedulesto exclude trees within the distances fromfoundations shown in Table 4, or

foundation depths shown in Table 5 with limitsagreed in the planting schedules to exclude treeswithin the zone of influence shown in Table 2.

Table 3 Minimum foundations depthsallowing for restricted new planting

Volume change potential Minimum depth [m]

High 1.5

Medium 1.25

Low 1.0

Table 4 No tree planting zone for minimumdepth foundations

Water demand No tree planting zone

High 1.0 x mature height

Moderate 0.5 x mature height

Low 0.2 x mature height

Table 5 Minimum foundations depths outsidezone of influence

Volume change potential Minimum depth [m]

High 1.0

Medium 0.9

Low 0.75

Planting schedules should be agreed with the localplanning authority before foundations are excavated.The landscape design should not compromise thestructural performance of the foundation.

(e) foundation depths related to newshrub planting

Shrubs have considerable potential to cause damageto foundations. Pyracantha, Cotoneaster andclimbers such as Ivy, Virginia Creeper and Wisteriacan be particularly damaging.

NHBC Standards • Chapter 4.2 Page 5 of 37


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DESIGN4.2 Building near trees

FOUNDATIONS (shrinkage soils)4.2 - D6 (continued)

(e) foundation depths relating to newshrub planting (continued)

Foundation depths relating to new shrub plantingshould be based on one of the following:

foundation depths shown in Table 3 in which casethere are no restrictions where shrubs may beplanted, or

foundation depths shown in Table 5 with limitsagreed in the planting schedules to exclude shrubswithin the distances from foundations shown inTable 6.

Table 6 No shrub planting zone for minimumdepth foundations

Volume change potential No shrub zone [m]

High 3.0

Medium 2.5

Low 2.0

Planting schedules should be agreed with the localplanning authority before foundations are excavated.The landscape design should not compromise thestructural performance of the foundation.

(f) strip or trench fill foundations in nonshrinkable soils overlying shrinkable soil

Non shrinkable soils such as sands and gravels mayoverlie shrinkable soil.

Foundations may be constructed on the overlyingnon shrinkable soil in accordance with Chapter 4.4'Strip and trench fill foundations' provided all of thefollowing conditions are satisfied, as illustrated inFigure 2:

consistent soil conditions exist across each plot.This should be confirmed by the site investigation

the depth of the non shrinkable soil is greaterthan 3/4 depth X, where X is the foundationdepth determined using Appendix 4.2-C or 42-D,assuming that all the soil is shrinkable

the thickness T of non shrinkable soil below thefoundation is equal to or greater than the width ofthe foundation B

the proposals are submitted to and approved byNHBC prior to work commencing on site.

Where any of the above conditions is not met,foundation depths should be determined as forshrinkable soil.

Figure 2 Foundations in non shrinkable soilsoverlying shrinkable soil

T equal to orgreater than B

B

depth Xdeterminedassumingsoil isshrinkable

acceptable foundationdepth

depthgreaterthan¾ X

no

n s

hri

nka

ble

so

ilsh

rin

kab

leso

il

(g) stepped foundationsWhere foundations are to be stepped to takeaccount of the influence of trees, hedgerows andshrubs they should be stepped gradually inaccordance with Chapter 4.4 'Strip and trench fillfoundations' with no step exceeding 0.5m (seeSitework clause S3(b)).

(h) foundations on or near sloping groundWhere the foundations are on or adjacent to slopingground greater than 1 in 7 (approximately 8°) andman-made slopes such as embankments and cuttingsthey should be designed by an Engineer (seeTechnical Requirement R5).

Items to be taken into account include:slope stability

potentially enhanced desiccation due to increasedrun-off and the de-watering effects of the slopeand vegetation.

Page 6 of 37 NHBC Standards - Chapter 4 2Effective: July 2003 April 2003 edition

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DESIGN

4.2 - D7 Foundations in shrinkable soils shallbe designed to transmit loads to theground safely and without excessivemovement


(a) strip foundationsStrip foundations up to 1.5m deep should beconstructed in accordance with the recommendationsof this Chapter and Chapter 4.4 'Strip and trench fillfoundations'. Depths should be determined inaccordance with Clause D6.

(b) trench fill foundationsTrench fill foundations up to 2.5m deep should beconstructed in accordance with the recommendationsof this Chapter and Chapter 4.4 'Strip and trench fillfoundations'. Depths should be determined inaccordance with Clause D6.

Reference should be made to Clause D8 to establishthe precautions necessary to cater for potential heave.

Trench fill foundations deeper than 2.5m will only beacceptable if they are designed by an Engineer (seeTechnical Requirement R5) taking account of allpotential movement of the soil on the foundationsand substructure.

The following will need to be taken into account iffoundations are to be deeper than 2.5m:

• foundation depths should be designed takingaccount of soil desiccation and arboricultural advice

• additional heave precautions may be necessary tocater for lateral and shear forces acting on largevertical areas of foundation

• instability of the trench sides can lead to seriousconstruction difficulties

• the foundation is dependent upon a high levelof workmanship and detailing:

- concrete overspill or overbreak in theexcavations can result in additional verticalforces being transmitted to the foundation

- construction joints will need to be detailedto take account of the increased lateral forces

- compressible material should be correctly placedto avoid excessive heave forces being applied tothe foundation.

(c) pier and beam foundationsPier and beam foundations should be designed by anEngineer (see Technical Requirement R5) andconstructed in accordance with the recommendationsof this Chapter and Chapter 4.5 'Raft, pile, pier andbeam foundations'.

Note: pier depths up to 2.5m may be derived fromClause D6. Pier depths greater than 2.5m requiresite specific assessment.


(d) pile and beam foundationsPile and beam foundations should be designed by anEngineer (see Technical Requirement R5) andconstructed in accordance with the recommendationsof this Chapter and Chapter 4.5 'Raft, pile, pier andbeam foundations'.


NHBC Standards - Chapter 4 2 Page 7 of 37April 2003 edition Effective: July 2003


DESIGN

FOUNDATIONS (shrinkable soils)4.2 - D7 (continued)

(e) raft foundations

Raft foundations should be designed by an Engineer(see Technical Requirement R5) and constructed inaccordance with the recommendations of thisChapter, Chapter 4.5 'Raft, pile, pier and beamfoundations' and the following conditions.

Raft foundations will only be acceptable where allof the following apply, as illustrated in Figure 3:

the foundation depth derived in accordance withClause D6 is 2.5m or less

the raft is founded on granular infill placed andfully compacted in layers in accordance with theEngineer's specification and to NHBC's satisfaction.The infill should not be less than 50% of thefoundation depth derived in accordance withClause D6 and should not exceed 1 25m. Siteinspections by the Engineer may be required byNHBC to verify the compaction of the fill

the infill extends beyond the edge of thefoundation by a distance equal to the naturalangle of repose of the infill plus 0.5m

the raft is generally rectangular in plan with a sideratio of not more than 2:1

NHBC is satisfied that the raft is sufficiently stiffto resist differential movements.

Figure 3 Requirements for raft foundations onshrinkable soils

raft foundationground level

1.25m max.depth (measuredin accordance withSitework clause S3)

level formation0.5 m fully 0.5 m

compactedinfill material

Page 8 of 37 NHBC Standards • C hapter 4.2Effective: July 2003 April 2003 edition

DESIGNING TOACCOMMODATE HEAVE4.2 - D8 Foundations, substructure and

services shall incorporate adequateprecautions to prevent excessivemovement due to heave

Heave can take place in a shrinkable soil when ittakes up moisture and swells after the felling orremoval of trees and hedgerows. It can also occurbeneath a building if roots are severed or if waterenters the ground from leaking drains, water servicesor changes in ground water conditions.


(a) vegetation surveyBefore the site is cleared, the location, heights andspecies of trees, hedgerows and shrubs on andadjacent to the site and which may affect proposedfoundations should be surveyed and recorded.

If the location of previously removed vegetation isnot known, local enquiries and reference to aerialphotographs may be necessary. Otherwise the designshould assume the worst conditions or an Engineer(see Technical Requirement R5) should be consultedto undertake a site specific design based on allrelevant information.

Where root growth is noted within shrinkable soiland where records are not available, an Engineer(see Technical Requirement R5) should be consultedto assess whether heave is likely.

(b) heave precautions for trench fill foundationsTrench fill foundations should be designed inaccordance with Clause D7. Any foundations deeperthan 2.5m should be designed by an Engineer(see Technical Requirement R5).

Heave precautions should be used:

• where the foundation is within the zoneof influence of trees (see Table 2), and

• where the foundation depth determined inaccordance with Clause D6 is greater than 1.5mbased on the appropriate tree height (see Figure 1).

Heave precautions for trench fill foundationsup to 2.5m should be in accordance with Siteworkclause S4(a).


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angle of repose ofinfill material


(c) heave precautions for pier and beamfoundations

Pier and beam foundations should be designed inaccordance with Clause D7.

Heave precautions for piers should be used:

where the foundation is within the zone ofinfluence of trees (see Table 2), and

where the foundation depth derived in accordancewith Clause D6 is greater than 1.5m based on theappropriate tree height (see Figure 1).

Heave precautions for pier and beam foundationsshould be in accordance with Siteworkclause S4(b).

(d) heave precautions tor pile andbeam foundations

Pile and beam foundations should be designed inaccordance with Clause D7.

Heave precautions should be used for piles andground beams in accordance with Sitework clauseS4(c). In addition the following should be taken intoaccount in the selection and design of piles:

piles should be designed with an adequate factorof safety to resist uplift forces on the shaft due toheave by providing sufficient anchorage below thedepth of desiccated soil. Slip liners may be used toreduce the uplift but the amount of reduction issmall, as friction between materials cannot beeliminated

piles should be reinforced for the length of thepile governed by the heave design

bored, cast-in-place piles are well suited to thisapplication. Most types have a straight-sided shaftbut some construction techniques produce acontoured shaft, similar to a screw profile, toincrease load capacity. The design should allow forthe enhanced tensile forces in such piles

driven piles are less well suited to this applicationand are difficult to install in stiff desiccated claywithout excessive noise and vibration. Most typesare jointed and, if these are to be used, the jointdesign should be capable of transmitting tensileheave forces

piles and ground beams should be designed takinginto account the upward force on the underside ofthe ground beams transmitted through thecompressible material or void former prior tocollapse (refer to manufacturer's data).

(e) suspended ground floorsSuspended ground floors should be used in allsituations where heave can occur within the areabounded by the foundations. This includes:

where the foundation depth derived in accordancewith Clause D6 is greater than 1.5m basedon the appropriate tree height (see Figure 1),unless NHBC is satisfied the soil is not dessicated

where ground floor construction is undertakenwhen surface soils are seasonally desiccated(i.e.during summer and autumn) unless NHBCis satisfied the soil is not desiccated.

The following types of suspended floor will beacceptable where there is potential for heave.

PRECAST CONCRETEA minimum void depth should be provided betweenunderside of beam and ground level as shown inTable 10 (see Sitework clause S4(d)).

TIMBERA minimum void depth should be provided betweenunderside of joist and ground level as shown in Table10 (see Sitework clause S4(d)). All sleeper wallsshould have foundations with depths derived inaccordance with Clause D6.

IN-SITU CONCRETEA minimum void depth should be provided betweenthe ground and the underside of slab as shown inTable 9 (see Sitework clause S4(d)). Where proprietarymaterials are used, they should be in accordance withMaterials clause M2 and the design should take intoaccount the upward force transmitted through thecompressible material or void former prior to collapse(refer to manufacturer's data).

(f) heave precautions for raft foundationsRaft foundations constructed in accordance withClause D7 should provide adequate protectionfrom heave.

(g) other foundationsAll foundations not covered in the above clauses, butspecifically designed for heave, should be designedby an Engineer (see Technical Requirement R5) takingaccount of the recommendations of this Chapter andsubmitted to NHBC for approval prior to workcommencing on site.

NHBC Standards - Chapter 4.2 Page 9 of 37April 2003 edition Effective: July 2003

DESIGN

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DESIGNING TOACCOMODATE HEAVE4.2 - D8 (continued)

(h) heave precautions for new drainsDrainage should be constructed in accordance withChapter 5.3 'Drainage below ground' with thefollowing additional precautions to guard against theeffects of heave.

• design gradients may need to be greater than theminimum gradients in Chapter 5.3 as these do notallow for possible ground movement. Wheresufficient falls to cater for the likely movementcannot be provided, alternative means of cateringfor the movement should be used, for exampletaking the excavation deeper and laying thepipework on granular bedding of suitablethickness to reduce the extent of potentialmovement

• a drainage system capable of accommodating thelikely movement should be used

• pipes and services passing through substructurewalls or trench fill foundations should be designedand detailed so as to cope with the potentialground movements shown in Table 7.

Table 7 Potential ground movement

Volume change potential Potential groundmovement [mm]

High 150

Medium 100

Low 50

Existing land drains should be maintained or diverted.Where the void beneath suspended floors is liable toflooding, drainage should be provided.

(i) paths and drivewaysDrives and pathways should be designed anddetailed to cater for the likely ground movement.

Further guidance is given in BS 5837.

PROVISION OF INFORMATION4.2 - D9 Designs and specifications shall

be produced in a clearlyunderstandable format and allrelevant information shall bedistributed to appropriate personnel

It is important that all relevant information neededfor the completion of the sitework is readilyavailable to all appropriate personnel.

All necessary dimensions and levels should beindicated and related to:

• at least one benchmark, and

• reference points on site.

Details should be provided with respect to:

• site investigation

• site survey including location and height of treesand hedgerows affecting the site

• site layout

• dimensions, type and depth of foundations

• soil volume change potential

• tree species (including existing, removedand proposed) using English names

• planting schedules

• original and final ground levels

• technical method statements including criticalsequences of construction

• location of services

• design of drainage system

• locations and detailing of:

- steps in foundations

- movement and construction joints

- ducts and services passing throughthe foundations.



DESIGN


MATERIALS STANDARDS4.2 - M1 All materials shall:

(a) meet the Technical Requirements

(b) take account of the design

Materials that comply with the design and theguidance below will be acceptable for buildingnear trees.

Materials used when building near trees shouldcomply with all relevant standards, including thoselisted below. Where no standard exists, TechnicalRequirement R3 applies (see Chapter 1.1'Introduction to the Standards and TechnicalRequirements').

References to British Standards and Codes of Practiceinclude those made under the Construction ProductsDirective (89/106/EEC) and, in particular, appropriateEuropean Technical Specifications approved by aEuropean Committee for Standardisation (CEN).

PROPRIETARY HEAVEMATERIALS4.2 - M2 Proprietary heave materials shall be

assessed in accordance with TechnicalRequirement R3

Where foundations and substructure could besubjected to heave, they should be protected byvoids, void formers or compressible materials inaccordance with the design.

Void formers consist of material that collapses toform a void into which the clay can swell reducingthe build up of load on the foundation.

Compressible material, such as low densitypolystyrene, compacts as the clay expands reducingthe build up of load on the foundation.

Each material should be used in accordance with therequirements of the relevant independent assessmentand the manufacturer's recommendations.


MATERIALS

SITEWORK4.2 Building near trees

SITEWORK STANDARDS4.2 - S1 All sitework shall:

(a) meet the Technical Requirements

(b) take account of the design

(c) follow established good practiceand workmanship

Sitework that complies with the design and guidancebelow will be acceptable for building near trees.

FOUNDATION DEPTHS4.2 - S2 Foundation depths shall be in

accordance with the design

A site plan should show the trees and hedgerowsthat affect the site together with the type, depth anddimensions of the foundations that are within theinfluence of those trees and hedgerows. Where treesor hedgerows are either not shown or are in differentpositions and there is shrinkable soil, it may benecessary to adjust the foundation depths on site.Foundation depths should be determined inaccordance with Design clause D6 or the foundationdepth calculator. If in doubt about any of theinformation either assume the worst conditionsor consult a suitably qualified Engineer.

An Engineer should be consulted where foundationdepths exceed 2.5m (see Technical Requirements R5).

Figure 4 NHBC foundation depth calculator

EXCAVATION FORFOUNDATIONS4.2 - S3 Excavation for foundations shall take

account of the design and be suitableto receive concrete


(a) measurement of foundation depthsFoundation depths should be measured on the centreline of the excavation.

Where ground levels are to remain unalteredfoundation depths should be measured from originalground level.

Where ground levels are reduced or increased (either inthe recent past or during construction) foundationdepths should be measured as shown in Figures 5 to 7.



SITEWORKBuilding near trees 4.2

Figure 5 Levels from which foundation depthsare measured where trees orhedgerows are to remain

Figure 7 Levels from which foundation depthsare measured where trees or hedgerowsare proposed

Use the lower of:a: foundation depth based on appropriate

tree height (see Figure 8)b: foundation depth based on mature

height of tree

Use the lower of:a: minimum foundation depth (see Table 8)b: foundation depth based on mature

height of tree

Figure 8 Tree height H to be used for particulardesign cases

Figure 6 Levels from which foundation depthsare measured where trees orhedgerows are removed

original ground level

Use the lower of:a: foundation depth based on appropriate

tree height (see Figure 8)b: minimum foundation depth (see Table 8)

This guidance should be used when:• deriving foundation depths when trees have been

removed (use tree height at time of removal)checking the appropriate level from which depthsshould be measured when trees remain and groundlevels are increased (use tree height at time ofconstruction relative to original ground level)determining whether heave precautions should beprovided (use tree height at time of construction).

Table 8 Minimum foundation depthsVolume change potential Minimum depth [m]

High

Medium

Low

1.0

0.75

NHBC Standards • Chapter 4.2 Page 13 of 37


•

•

0.9

SITEWORK

EXCAVATION FORFOUNDATIONS4.2 - S3 (continued)

(b) stepped foundationsFor stepped foundations, the relevantrecommendations of Chapter 4.4 'Strip and trenchfill foundations' should be followed with theadditional precaution that the maximum step heightshould not exceed O.5m as shown in Figure 9.

On sloping ground, foundation trenches can begradually stepped so that the required foundationdepth is reasonably uniform below ground level.

Figure 9 Stepped foundations

ground level

foundation depth

line of trench bottom

step not greaterthan 0.5m

(c) trench bottomsWhere trench bottoms become excessivelydried or softened due to rain or ground water, theexcavation should be re-bottomed prior to concreting.

Some root activity may be expected below thedepths determined in accordance with Designclause D6. However, if significant quantities of rootsare unexpectedly encountered in the base of thetrench, the excavation should be deepened orconsult an Engineer.

HEAVE PRECAUTIONS4.2 - S4 Heave precautions shall be

incorporated into foundations andsubstructure in accordance withthe design

The following details show the minimumrequirements for common foundation types. Theyapply to all foundations within the zone of influenceof trees which are to remain or be removed.

Correct placement of heave materials is essential toensure the foundations and substructure areadequateley protected from heave forces.

(a) heave precautions for trench fill foundationsHeave precautions should be provided as shownin Figure 10.

Compressible material should be provided against theinside faces of all external wall foundations greaterthan 1.5m deep based on the appropriate tree height(see Figure 8).

No compressible material is required against the facesof internal foundations.

Heave precautions are not required for proposed treesas the soil has not been desiccated and thereforeheave cannot take place.

Figure 10 Heave precautions for trench fillfoundations up to 2.5m deep

It is essential that:

• compressible material is provided to the entire area shown,and

• the foundation excavation has a vertical face. Where theexcavation is battered or if there is overbreak or concreteoverspill it may be necessary to consult an Engineer.

Trench fill foundations deeper than 2.5m will onlybe acceptable where they are designed by anEngineer (see Technical Requirement R5).

(b) heave precautions for pierand beam foundations

Heave precautions should be provided asshown in Figure 11.

Compressible material should be provided against allfaces of the pier foundation which are greater than1 5m deep based on the appropriate tree height (seeFigure 8).

Page 14 of 37 NHBC Standards • Chapter 4.2



Void'free Table 9 or 10) 450mm

maxbackfill

verticalface tofoundation

compressible material-(see Table 9)

500mm

SITEWORK

A void, void former or compressible material shouldbe provided below all ground beams.

Compressible material or a void former should alsobe provided against the inside faces of externalground beams unless NHBC is satisfied that the soil,at this level, is not desiccated.

Heave precautions are not required for proposedtrees as the soil has not been desiccated and heavecannot take place.

Figure 11 Heave precautions for pier andbeam foundations

void (see Table 9 or 10)

compressible materialor void former toinside face of externalground beams(see Table 9)

compressible materialor void former beneathground beams(see Table 9)

500mm

backfillembedment ofanchorage bars tobe 40 bar diametersor designed by anEngineer (see TechnicalRequirement R5)compressible materialto sides of piers(see Table 9)

It is essential that heave material is provided to the entireareas shown. Particular care should be taken to ensurethat the full width of the ground beam is protected.

(c) heave precautions for pile andbeam foundations

Heave precautions should be provided asshown in Figure 12.

A void, void former or compressible material shouldbe provided below all ground beams.

Compressible material or a void former should alsobe provided against the inside faces of externalground beams unless NHBC is satisfied that the soil,at this level, is not desiccated.

Heave precautions are not required for proposedtrees as the soil has not been desiccated and heavecannot take place.


Figure 12 Heave precautions for pile andbeam foundations

Void(see Table

9 or 10) backfill

compressible materialor void former toinside face of externalground beams(see Table 9)

compressible materialor void formerbeneath groundbeams (see Table 9)

pile length toEngineer's design

embedment of pile tensionreinforcement to be 40 bardiameters or designed byan Engineer (see TechnicalRequirement R5)

optional rigidslip liner

It is essential that heave material is provided to the entireareas shown. Particular care should be taken to ensure thatthe full width of the ground beam and the areas aroundthe piles are protected.

(d) minimum void dimensionsVoids should be provided to accommodate movementin accordance with Tables 9 and 10.

Table 9 Minimum void dimension forfoundations, ground beams andsuspended in-situ concrete ground floors

Volumechangepotential

HighMediumLow

Against sideof foundation

and ground beam

Void dimension[mm]1

3525

0

Underground beam

and suspendedin-situ concrete

ground floor

Void dimension[mm]1

15010050

Note:1 For compressible material the void dimension is the

amount the material should be able to compress toaccommodate heave. The actual thickness ofcompressible material required should be establishedfrom the manufacturer's recommendations and isgenerally in the order of twice the void dimensionshown and;

for void formers the void dimension is the remainingvoid after collapse. The actual thickness of voidformer required should be established from themanufacturer's recommendations.

NHBC Standards - Chapter 4.2 Page 15 of 37


SITEWORK

HEAVE PRECAUTIONS4.2 - S4 (continued)

(d) minimum void dimensions (continued)

Table 10 Minimum void dimensions under precastconcrete and timber ground floors

Soil heavepotential

HighMediumLow

Precastconcrete

Void dimension[mm]1

225175125

Suspendedtimber

Void dimension[mm]2

300250200

Note:1 Measurement from underside of beam to ground level

(includes 75mm ventilation allowance).

2 Measurement from underside of joist to ground level(includes 150mm ventilation allowance).

DRAINAGE4.2 - S5 Drainage shall be in accordance

with the design and allow forground movement

Drainage construction should be in accordance with thedesign and the relevant recommendations of Chapter5.3 'Drainage below ground' should be followed.

Additional items to take into account include:

falls should be sufficient to cater for possibleground movement or alternative means should beused to reduce the extent of potential movement,for example by taking the excavation deeper andlaying the pipework on granular bedding ofsuitable thickness

a drainage system capable of accommodating thelikely movement should be used

pipes passing through substructure walls or trenchfill foundations should have sufficient clearance totake account of the potential ground movementindicated in Table 11.

Table 11 Minimum allowance for potentialground movement

Volume change potential

High

Medium

Low

Potential groundmovement [m]

150

100

50


Existing land drains should be maintained ordiverted. Where the void beneath suspended floorsis liable to flooding, drainage should be provided.



APPENDIX 4.2-A

Statutory referencesThe following table lists references to building legislation and associated documents applicable at April 2003.

Clause

D4D5D6D7

Subject

Foundation design

Statute

BuildingRegulations

A1/2

Building 2

Standards(Scotland)

Part C

Building 3

Regulations(N Ireland)

Part D

Isle of Man 4

Regulations

A1/2

Approved documents to the Building Regulations 2000 for England and Wales and all published amendments.Building Standards (Scotland) Regulations 1990 and all published amendments.Building Regulations (Northern Ireland) 2000 and all published amendments.Building Regulations 2000 for Isle of Man and all published amendments.

NHBC Standards - Chapter 4.2 Page 17 of 37April 2003 edition Effective July 2003


1

1234


Water demand and mature height of treesTable 12

Broad leafed treesWater Mature heightdemand Species [m]High Elm

English 24Wheatley 22Wych 18

Eucalyptus 18Hawthorn 10Oak

English 20Holm 16Red 24Turkey 24

PoplarHybrid black 28Lombardy 25White 15

WillowCrack 24Weeping 16White 24

Moderate Acacia false 18Alder 18Apple 10Ash 23Bay Laurel 10Beech 20Blackthorn 8Cherry

Japanese 9Laurel 8Orchard 12Wild 17

ChestnutHorse 20Sweet 24

Lime 22Maple

Japanese 8Norway 18

Mountain Ash 11Pear 12Plane 26Plum 10Sycamore 22Tree of Heaven 20Walnut 18Whitebeam 12

Low Birch 14Elder 10Fig 8Hazel 8Holly 12Honey Locust 14Hornbeam 17Laburnum 12Magnolia 9Mulberry 9Tulip tree 20

Coniferous treesWaterdemandHigh

Moderate

SpeciesCypress

Lawson'sLeylandMonterey

CedarDouglas firLarchMonkey PuzzlePineSpruceWellingtoniaYew

Mature height[m]

182020

2020201820183012

Note:

1. Where hedgerows contain trees, their effectsshould be assessed separately. In hedgerows, theheight of the species likely to have the greatesteffect should be used.

2. Within the classes of water demand, species arelisted alphabetically; the order does not signifyany gradation in water demand.

3. When the species is known but the sub-species isnot, the greatest height listed for the speciesshould be assumed.

4. Further information regarding trees may beobtained from the Arboricultural Association orthe Arboricultural Advisory and InformationService (see Appendix 4.2-G).



APPENDIX 4.2-B

Sharon Smith

Sharon Smith

APPENDIX 4.2-CBuilding near trees 4.2

Foundation Depth ChartsTable 13 Determination of D/H Value



APPENDIX 4.2-C4.2 Building near trees

Foundation depth charts (continued)Chart 1 Soils with HIGH volume change potential: Modified Plasticity Index 40% or greater

(see Design clause D5(b))

D/H

0 0.2 0.4 0.6 0.8 1.0 1.20

0.5

1.0

1.5

2.0

2.5Tree water demands

Broad leafed trees

High Moderate Low

Coniferous trees High

— — — Moderate



— — —

APPENDIX 4.2-CBuilding near trees 4.2

Chart 2 Soils with MEDIUM volume change potential: Modified Plasticity Index between20% and less than 40%(see Design clause D5(b))

D/H

0 0.2 0.4 0.6 0.8 1.0 1.2

Fou

ndat

ion

dept

hs (m

)

0

0.5

1.0

1.5

2.0

2.5

Tree water demandsBroad leafed trees

High Moderate Low

Coniferous trees High Moderate


APPENDIX 4.2-C4.2 Building near trees

Foundation depth charts (continued)Chart 3 Soils with LOW volume change potential: Modified Plasticity Index 10 to less than 20%

(see Design clause D5(b))

D/H

0 0.2 0.4 0.6 0.8 1.0 1.2

Fou

ndat

ion

dept

hs (m

)

0

0.5

1.0

1.5

2.0

2.5Tree water demands

Broad leafed trees

High Moderate Low

Coniferous trees

High Moderate




Foundation depth tablesTable 14 - HIGH shrinkage soil and HIGH water demand tree



APPENDIX 4.2-D

APPENDIX 4.2-D4.2 Building near trees

Foundation depth tables (continued)Table 15 - HIGH shrinkage soil and MODERATE water demand tree



APPENDIX 4.2-DBuilding near trees 4.2

Table 17 - MEDIUM shrinkage soil and HIGH water demand tree

NHBC Standards - Chapter 4 2 Page 25 of 37April 2003 edition . Effective: July 2003


Foundation depth tables (continued)Table 18 - MEDIUM shrinkage soil and MODERATE water demand tree

Table 19 - MEDIUM shrinkage soil and LOW water demand tree

Page 26 of 37 NHBC Standards - Chapter 4.2Effective: July 2003 April 2003 edition

APPENDIX 4.2-DBuilding near trees 4.2

Table 20 - LOW shrinkage soil and HIGH water demand tree



Foundation depth tables (continued)Table 21 - LOW shrinkage soil and MODERATE water demand tree

Table 22 - LOW shrinkage soil and LOW water demand tree


APPENDIX 4.2-E

Climate zonesFigure 13 Reductions in foundation depth due to climate variationsThe foundation depth may be reduced by the amounts shown on the map for each climatic zone (seeDesign clause D5(e)). Where it is unclear which zone applies, the lower reduction value should be used.

0.50m (500mm)

0.45m (450mm)

0.40m (400mm)

0.35m (350mm)

0.30m (300mm)

0.25m (250mm)

0.20m (200mm)

0.15m (150mm)

0.10m (100mm)

0.05m (50mm)




APPENDIX 4.2-F4.2 Building near trees

Damage to trees by construction work derived from BS 5837:1991In order to avoid unacceptable damage as a result of construction activities, an area around each remaining treeshould be protected from disturbance by fencing. This should not be removed or breached during constructionoperations without prior consultation with an arboricultural specialist. The fencing should protect as large an areaaround the tree as possible after consideration of all construction operations in its vicinity. The minimum distance leftto be undisturbed around the tree is given in the table below.

Protection of trees: minimum distances for protective fencing around trees

Tree age

Young trees(age less than life expectancy)

Young trees

Middle age treesto : life expectancy)

Middle age trees

Mature trees

Mature trees and overmature trees

Tree vigour

Normal vigour

Low vigour

Normal vigour

Low vigour

Normal vigour

Low vigour

Trunk diameter

mm< 200

200 to 400> 400

< 200200 to 400

> 400

< 250250 to 500

> 500

< 250250 to 500

> 500

< 350350 to 750

> 750

< 350350 to 750

> 750

Minimum distance

m2.03.04.0

3.04.56.0

3.04.56.0

5.07.5

10.0

4.06.08.0

6.09.0

12.0

Note:

1. It should be emphasised that this table relates to distances from centre of tree to protective fencing.Otherconsiderations, particularly the need to provide adequate space around the tree including allowances forfuture growth and working space will usually indicate that the structures should be further away.

2. With appropriate precautions, temporary site works can occur within the protected area, e.g. for accessor scaffolding.

3. If it is deemed acceptable for construction works to occur closer than the minimum distance, the distancecan be reduced by up to one third on one side only. If distances are reduced in this way, a correspondingincrease in distances should be made in other directions.


APPENDIX 4.2-FBuilding near trees

As an alternative, the fencing may be erected below the outermost limit of the branch spread, or at adistance equal to half the height of the tree, as illustrated below. The distance by this method will usually besignificantly greater than the distances in the table above.

Alternative location for protective fencing

Trenching along radii to minimise damage

If it proves essential for a service trench to be taken closer to a tree than the minimum distance in the abovetable, root damage can be minimised by either:

• thrust boring a hole for the service, or

• radial trenching and tunnelling as illustrated below.



Fence

Branch spread use whichever is greater Half height

Fence

Minimum1.0m

Trench

Sleeve

Service

APPENDIX 4.2-F4.2 Building near trees

Damage to trees by Construction work derived from BS 5837 : 1991 (continued)If building closer than the distances recommended in the table below, precautions should be taken to allow forfuture growth of the tree.

Minimum distance (m) between centre of tree and structure to allow for future tree growth

Type of structure

Buildings and heavily loaded structures

Lightly loaded structures such as garages, porches etc.

Drains and underground services<1m deep>1m deep

*Masonry boundary walls

*ln-situ concrete paths and drives

*Paths and drives with flexible surfaces or paving slabs

*These distances assume that some movement and minor damagewill generally avoid all damage is given in brackets.

Mature height of tree

Up to 8m

-

-

0.5

_

(0.5)

(0.7)

8m to 15m

0.5

0.7

1.51.0

0.5(1.0)

0.5(1.0)

0.5(1.5)

over 15m

1.2

1.5

3.02.0

1.0(2.0)

1.5(2.5)

1.0(3.0)

might occur. Guidance on distances which

Further information can be obtained by reference to BS 5837.


APPENDIX 4.2-GBuilding near trees 4.2

Information sources and acknowledgements

INFORMATION SOURCESFurther recommendations and information can be obtained from:

Publications

BS 1377 'Methods of test for soils for civil engineering purposes'BS 5837 'Guide for trees in relation to construction'BS 5930 'Code of practice for site investigations'BRE Digests 240, 241 and 242 'Low rise buildings on shrinkable clay soils', parts 1, 2 and 3BRE Digest 298 'The influence of trees on house foundations in clay soils'BRE Digest 412 'Desiccation in clay soils'

Tree Recognition - A Pocket Manualby Ian Richardson and Rowena Gale, Richardson's Botanical Identifications,49/51 Whiteknights Road, Reading, Berks RG6 7BB

Field Guide to the Trees of Britain and Northern Europeby Alan Mitchell, Harper Collins, Glasgow

Geological survey mapsobtainable from British Geological Survey, Nicker Hill, Keyworth, Nottingham NG12 5GGTel: 0115 936 3100; www.bgs.ac.uk

Tree root damage to buildingsVol.1 Causes, Diagnosis and RemedyVol. 2 Patterns of Soil Drying in Proximity to Trees on Clay Soilsby P G Biddle, Willowmead Publishing, Wantage OX12 9JA

Organisations

Arboricultural AssociationAmpfield House, Ampfield, nr. Romsey, Hants SO51 9PATel: 01794 368717; www.trees.org.uk

Arboricultural Advisory and Information ServiceForest Research Station, Alice Holt Lodge, Wrecclesham, Farnham, Surrey GU10 4LHTel: 01420 22022; www.treehelp.info(Tree Helpline telephone no. 0906 516 1147)

Institution of Civil Engineers1-7 Great George Street, London SW1P 3AATel: 020 7222 7722; www.ice.org.uk

Institution of Structural Engineers11 Upper Belgrave Street, London SW1X 8BHTel: 020 7235 4535; www.istructe.org.uk

ACKNOWLEDGEMENTSNHBC gratefully acknowledges the help given by authoritative organisations and individuals in the preparation ofthis Chapter, particularly:

Building Research EstablishmentDr P G Biddle Arboricultural Consultant



APPENDIX 4.2-H4.2 Building near trees

Worked exampleHow to determine foundation depths from the Charts in Appendix 4.2-C or the Tables in Appendix 42-D.

Step Ref Example

1 Determine the volumechange potential of thesoil. Ensure the siteinvestigation includesrepresentative samplingand testing.

D5(b) Site at Oxford, building near a Lombardy Poplar (to be retained) and aSycamore (to be removed).

From laboratory tests,

Plasticity Index, Ip = 36%.Test results also report that 100% of particles are smaller than425um. Therefore,

modified Plasticity Index. I'p = 36 x 100 = 36%.100

From Table 1, Volume change potential = Medium

(in the absence of tests assume high volume change potential)

This example is typical of Oxford Clay. More than 35% of theparticles are smaller than 60 m m and therefore the soil isshrinkable. 100% of the particles are smaller than 425um andtherefore the I'p is the same as the Ip.

A typical Boulder Clay also has more than 35% of particlessmaller than 60um and is therefore also shrinkable. However, itmay have only 80% of its particles smaller than 425um in whichcase the I'p is 80% of the Ip.

A typical clayey sand may have less than 30% of its particlessmaller than 60um in which case the soil would be nonshrinkable.

2 Establish the species,mature height and waterdemand of all trees andhedgerows within theirinfluencing radii.

D5(c)andD5(d)

Lombardy Poplar Sycamore

From Appendix 4.2-BMature height = 25mWater demand = High

From Appendix 4.2-BMature height = 22mWater demand = Moderate

3 Plot the trees andhedgerows relative to thefoundations and drawtheir zones of influence todetermine which trees willaffect the foundation design.Use a scaled plan.

D5(c)

zone of influenceof Sycamore0.75x22 = 16.5m


APPENDIX 4.2-HBuilding near trees 4.2

S tep Ref Example

4 Establish the appropriatetree height H to use.Always use the mature heightfor remaining and proposedtrees and hedgerows. Theappropriate height to use forremoved trees andhedgerows depends on theactual height when they areremoved.

D5(d) Lombardy Poplar Sycamore

Tree to remain. Therefore,H = Mature height

= 25m

Tree to be removedMature height = 22mActual height = 15mActual height greater than50% mature height. Therefore,H = Mature height

= 22m

5 Measure the distance Dfrom the centre of the treesor hedgerows to the face ofthe foundation.

D6(c) Lombardy Poplar Sycamore

Distance D = 10mfrom foundation

Distance D = 8mfrom foundation

6 Select Steps 6C(a) and (b) ifusing Charts in Appendix4.2-C to derive depths orselect Step 6T if usingTables in Appendix 4.2-Dto derive depths. Alternativelythe NHBC foundation depthcalculator may be used (seeSitework clause S2).

6C(a)

Calculate D/Hi.e. distance D from faceof foundation (Step 5)divided by the appropriatetree height H (Step 4).Alternatively D/H can beobtained from Table 13 inAppendix 4.2-C.


D = 10 = 0.4H 25

D = 8 =0.36H 22

6C(b)

Determine foundationdepth using the Charts inAppendix 4.2-C as follows:


In this example the volumechange potential isMedium, then from Chart 2for broadleafed high waterdemand trees at D = 0.4,

HFoundation depth = 2.33m

In this example the volumechange potential is Medium,then from Chart 2 forbroadleafed high waterdemand trees at D = 0.36,

HFoundation depth = 1.50m

The Lombardy Poplar is the tree requiring the greaterdepth (2.33m)

Volumechangepotential

Chartnumber

High 1

Medium 2

Low 3


APPENDIX 4.2-H4.2 Building near trees

Worked example (continued)

Step Ref Example

6T Determine foundation depth usingthe Tables in Appendix 42-D as follows:


Volumechange

potential

Treewater

demand

Tablenumber

In this example thevolume change potentiais Medium and thewater demand is High,then from Table 17,for broad leafed highwater demand trees atD = 10m and H = 25m,Foundation depth =2.33m (by interpolation)

The Lombardy Poplar is the tree requiring thegreater depth (2.33m)

In this example thevolume change potentialis Medium and the waterdemand is Moderate,then from Table 18, forbroad leafed moderatewater demand trees atD = 8m and H = 22m,Foundation depth =1.50m

7 Adjust the depth according to theclimatic zone. A reduction may bemade for distance north and west ofLondon but the final depth should notbe less than the minimum given in eachChart and Table.

D5(e) Oxford is between 50 and 100 miles NW ofLondon. From Appendix 4.2-E, a reduction of0.05m is permitted.Final foundation depth = 2.33 - 0.05 = 2.28m

8 Check that the recommendations ofthis Chapter have been met for:

Acceptable foundation types

New planting(including shrubs)

Non shrinkable soil overlyingshrinkable soil

Variations in foundation depths

Foundations on sloping ground

Precautions against heave

(including suspended floors)

Measurement of foundation depths

Foundation trench bottoms

Precautions for drainage

D6(a)

D6(d),D6(e)

D6(f)

D6(g),S3(b)

D6(h)

D8.S4

S3(a)

S3(c)

S5

Note:

The above process may be repeated to allow the foundation to be stepped as its distance from the trees increases.



HighHighModerateLow

141516

MediumHighModerateLow

171819

LowHighModerateLow

202122

LIST OF CHAPTERS AND INDEXBuilding near trees 4.2

LIST OF CHAPTERSPART 1 GENERAL INFORMATION1.1 Introduction to the Standards and Technical

Requirements1.2 Not allocated1.3 Not allocated1.4 Cold weather working

PART 2 MATERIALS2.1 Concrete and its reinforcement2.2 Not allocated2.3 Timber preservation (natural solid timber)

PART 3 NOT ALLOCATED

PART 4 FOUNDATIONS4.1 Land quality - managing ground conditions4.2 Building near trees4.3 Not allocated4.4 Strip and trench fill foundations4.5 Raft, pile, pier and beam foundations4.6 Vibratory ground improvement techniques

PART 5 SUBSTRUCTURE AND GROUNDFLOORS

5.1 Substructure and ground bearing floors5.2 Suspended ground floors5.3 Drainage below ground

PART 6 SUPERSTRUCTURE (excluding roofs)6.1 External masonry walls6.2 External timber framed walls6.3 Internal walls6.4 Timber and concrete upper floors6.5 Steelwork support to upper floors and

partitions6.6 Staircases6.7 Doors, windows and glazing6.8 Fireplaces, chimneys and flues

PART 7 ROOFS7.1 Flat roofs and balconies7.2 Pitched roofs

PART 8 SERVICES AND INTERNAL FINISHING8.1 Internal services8.2 Wall and ceiling finishes8.3 Floor finishes8.4 Finishings and fitments8.5 Painting and decorating

PART 9 EXTERNAL WORKS9.1 Garages9.2 Drives, paths and landscaping

PART 10 NOT ALLOCATED

BBroad leafed trees 18, 23, 24, 25CClimate 4, 29Compressible materials 11, 14, 15Coniferous trees 18, 23, 24, 25DDamage to trees 30, 31Depth charts 19-22Depth Tables 23-28Drainage 10, 16, 21, 34, 35EExcavation 12FFoundation depths 5, 12, 19, 21, 26, 32Foundation types 4, 21HHeave 3, 8, 9, 14Heave precautions 8, 9, 10, 11, 14, 21M

Plasticity Index 3, 19NNew planting 5, 6, 21PPier and beam foundations 4, 7, 9, 14, 15Pile and beam foundations 4, 7, 9, 14, 15

Plasticity Index 3, 19Protection of trees 2, 19, 34, 35RRaft foundations 4, 8, 9, 16SShrinkable soils 3,6,21,26Shrubs 5, 6Sloping ground 6, 21Soil classification 3Stepped foundations 6,14,21Strip foundations 4, 6, 7Suspended ground floors 9TTree heights 4, 13, 18, 20, 22. 35Tree species 18Trench bottoms 2,4,21Trench fill foundations 4, 7, 8, 14VVoid formers 11, 14, 15Volume change potential 3,19WWater demand 3, 18, 23, 24, 25ZZone of influence 3, 8, 34



INDEX

4.24.2

chapter 4.2 building near trees

Documents

technical

local planning

volume change

external ground

modified plasticity

sitework clause

atterberg

deriving foundation